The method of electrophotography prepares a copy by forming an electrostatic latent image on the photoconductive surface of a photosensitive member, by developing the electrostatic latent image to form a toner image, and by fixing the toner image either directly or after the toner image has been transferred to a transfer member. In case the electrostatic latent image is to be developed, a magnetic brush developing method is usually conducted, in which case a two-component system developer, which is a mixed powder comprising a non-magnetic toner and a carrier such as iron powder is used as a developing agent. When using this magnetic brush developing method with the two-component system developer, the carriers and the toner are mixed in a predetermined ratio and are frictionally charged so that the toner is charged with a predetermined polarity until it sticks to the surface where the electrostatic latent image is formed. Therefore, the magnetic brush developing method has the advantage that the transfer is feasible. However, the magnetic brush developing method requires means for mixing and frictionally charging the toner and the carriers to a satisfactory extent. Since only the toner is consumed during the development, moreover, the developing method requires the so-called "toner control device", i.e., a toner concentration monitoring device for maintaining a uniform toner concentration. As a result, the developing device needs to be bigger and have a more complicated construction. Moreover, since the carriers become exhausted after extended periods of time, the developing method has the problem that it is necessary to replace the carriers.
The developer, therefore, that has been used in recent years is a one-component system non-chargeable magnetic toner which is comprised essentially of magnetic powder and resin. A developing method using that magnetic toner, has been a method which is disclosed in Japanese Patent Publication No. 56-2705. The developing method disclosed selectively deposit toner on the surface of the electrostatic latent image by using as magnetic toner the conductive and magnetic toner which has an electric resistance of about 10.sup.4 to 10.sup.11 .OMEGA..multidot.cm when an electric field of a direct current of 100 V/cm is applied, by forming a magnetic toner carrier of a conductive sleeve, by electrically coupling the back of an electrostatic latent image carrier and the conductive sleeve thereby to form a electrically conductive path through the conductive and magnetic toner layer between the surface of the electrostatic latent image carrier and the conductive sleeve, by collecting at the leading end of a toner brush the charges which have a polarity opposite to that of the electrostatic latent image induced on the conductive sleeve by the charges of the electrostatic latent image, and by relatively moving the conductive sleeve and the electrostatic latent image carrier so that the coulomb force generated between the charges at the leading end of the toner brush and the charges of the electrostatic latent image may overcome the magnetic attraction generated by a permanent magnet roll disposed inside of the conductive sleeve.
The developing method thus disclosed can be applied to the so-called "CPC method", by which the toner image obtained by the development is fixed to directly prepare a copy, because the toner used has conductivity. However, the method intrinsically desired is not the aforementioned CPC method for a direct record but for an indirect record, i.e., the so-called "PPC method", by which a photosensitive member acting as a master is repeatedly used for development so that the developed toner image can be transferred to ordinary paper having a low electric resistance.
If the aforementioned developing method is applied to the PPC method, the development is satisfactorily conducted because the electric resistance of the toner is low, but toner splash and reduction of the transfer electric field due to leakage take place at the transferring step to make the transferred image obscure. This raises the difficulty that it is difficult to apply the magnetic toner in the PPC method.
In order to overcome the difficulty in transfer, a method has been proposed in which the development is conducted by using insulating magnetic toner having a higher electric resistance than the magnetic toner and with which the transfer is then conducted. According to the method proposed, a device is required for raising the developing efficiency of the toner because the developability generally becomes worse as the electric resistance of the magnetic toner becomes higher. As the device therefor, there is disclosed in Japanese Patent Laid-Open No. 53-129639, for example, a method in which the sleeve of a developing device is rotated to restrict the velocity difference between the sleeve and a photosensitive member to within a predetermined range thereby enhancing the developing efficiency. In the method disclosed in Japanese Patent Laid-Open No. 53-31136, on the other hand, the intent is to enhance the developability of the insulating magnetic toner by improving the aforementioned sleeve rotating system to increase the moving velocity of the toner. More specifically, the method disclosed affects the development by using insulating magnetic toner having an electric resistance not lower than 10.sup.12 .OMEGA..multidot.cm when an electric field of a direct current of 10,000 V/cm is applied, by adhering the conductive particles to the surface of the toner within such a range as does not reduce the electric resistance, by bringing the toner and the electrode into electric contact through the high-speed rotations of the sleeve and/or the magnet roll thereby to charge the toner with conductive particles, and by moving the toner with conductive particles to the surface of the latent image at a high speed not lower than 10 cm/sec. In case the development is conducted by the disclosed method, however, the toner comes into contact with the latent image surface at a high speed. If the parting velocity of the toner from the latent image is increased, on the other hand, the toner charged by the electrode sticks to the latent image surface due to the coulomb force, but is recovered from the latent image surface because the cleaning effect of the magnetic brush becomes strong, thereby inviting a phenomenon where the developing efficiency is degraded. According to the disclosed method, moreover, in the case where the toner is moved by the rotation the sleeve only, the doctor section cannot uniformly regulate the toner and becomes liable to be clogged with a cluster of toner or dust thereby forming streaks from insufficient development due to shortage of the toner on the sleeve.
In order to eliminate those defects, therefore, there has been proposed in Japanese Patent Publication No. 57-12148 and Japanese Patent Laid-Open No. 55-126266 a method in which the moving velocity of the toner is not higher than 10 cm/sec and in which the sleeve and the magnet roll are rotated in an identical direction. According to the method disclosed, the deterioration of the toner movement on the sleeve is eliminated and the charging efficiency of the electrode is improved so that the insulating non-chargeable magnetic toner can be applied to the PPC method although it has been accepted as being difficult.
Thanks to the methods thus far described, the PPC method of the non-chargeable magnetic toner has reached practical application. However, the insulating non-chargeable magnetic toner generally cannot be charged even with the aforementioned development devices, and the surface potential of the photosensitive member has to be set to be higher than the ordinal level by several hundred volts in order that the toner may achieve the charge necessary for the development. In case the surface potential of the photosensitive member is to be dropped, on the other hand, a photosensitive zinc oxide having a short lifetime has to be used. In any case, therefore, the lifetime of the photosensitive member is shortened, which raises the running cost of the copies.
This leads to the proposal of triboelectric magnetic toner with a chargeability which is prepared by adding a charge control agent to the magnetic toner and by charging the toner in advance with a positive or negative polarity. This proposal is directed, as has been disclosed in Japanese Patent Laid-Open Nos. 55-48754, 57-45555, 57-45556 and 55-45557, to a method in which the magnetic toner is charged in advance for the development by adding a substance having a strong property as an electron acceptor or an electron donor to the inner and/or outer side of the magnetic toner.
If such charge control agent is added to the magnetic toner, this toner is charged by the frictional charging action between the toner particles, between the toner and the sleeve or between the toner and the doctor to achieve a charge substantially equal to that of the two-component system toner so that the photosensitive member can be developed under the condition where the surface potential of the photosensitive member is similar to that of the two-component system developer of the prior art. With the chargeable magnetic toner, however, the frictional chargeability of the toner particles is increased as the charge of the toner is improved, and the toner particles are liable to become charged and agglomerate on the sleeve so that the agglomerated toner deposits upon the toner regulating plate (i.e., the doctor blade) and forms streaks where development is incomplete due to a shortage of toner on the sleeve.